1. Field of the Invention
The present invention is related to an automatic gain amplifier, and more particularly to an automatic gain amplifier used in a biometric sensing device.
2. Background Information
In biometric imaging devices, for example optical-based or electrical properties-based fingerprint scanning devices, a user places a finger on a fingerprint sensor. The fingerprint sensor scans the fingerprint and generates an analog or digital signal that is representative of the scanned fingerprint. According to at least one imaging technique, a grayscale image is produced of the scanned fingerprint. Each pixel of the grayscale image has a value between 0 and 255xe2x80x94a value of 0 value representing a xe2x80x9cblackxe2x80x9d pixel and a value of 255 representing a white pixel. Based on this grayscale image, biometric information is extracted.
FIG. 1 shows a diagram of a known fingerprint imaging device 2. In normal operation, the fingerprint imaging device uses techniques derived from Coulombs law to determine the location of ridges and valleys in a fingerprint surface. By modeling each sensing element on the sensor as one plate in a capacitor and the finger surface (that is, the ridges and valleys) as the second plate in the capacitor, it is possible to measure a relative distance between the ridges and valleys to construct the fingerprint.
As is shown in FIG. 2, the fingerprint imaging device 2 is typically embodied in biometric sensor chip 4, which consists of an m by n array of sensing elements or capacitive plates 6.
A drawback to known biometric sensing devices, and in particular the imaging techniques employed, is that from individual to individual, and indeed, from situation to situation, the characteristics of the biometric, that is, the finger, can vary greatly. For instance, the moisture content of the finger may vary, as can the relative distance between ridges and valleys. Because these parameters vary, the resultant biometric image may not have the requisite image clarity needed when the biometric sensing device is deployed in a highly sensitive environment.
An automatic gain amplifier is disclosed that dynamically improves the quality of scanned biometric information. According to one embodiment, an image distribution of the scanned biometric is generated. Next, areas of higher image distribution are identified. An iterative process of adjusting the gain of a biometric sensor device, for instance, a capacitive sensing element, is employed until an optimum separation of the areas of higher image distribution is achieved. Once the optimum separation is achieved, the biometric sensing device analyzes the resulting image so that biometric information can be detected with improved image clarity and less interference with noise, such as dust and changing biometric parameters. Electronic circuitry and software for implementing the methods and apparatuses of the invention are disclosed.